Log periodic type antenna mounted on ground plane and fed by tapered feed



Aug. 1963 A. F. WICKERSHAM, JR.. ETAL 3,101,474

LOG PERIODIC TYPE ANTENNA MOUNTED 0N GROUND PLANE AND FED BY TAPERED FEED Filed Nov. 25, 1960 2 Sheets-Sheet 1 VSWR (REF TO 50 2.0 A 95 J '5 l l 1 f V\ INVENTORS .0 ARTHUR F. WICKERSHAM JR.

500 I000 1500 2000 R085 L. BELL FREQUENCY (MC/SEC) ATTOR N EY Aug. 20, 1963 A. F. WICKERSHAM, JR.. ETAL 3,101,474

LOG PERIODIC TYPE AN NA MOUNTED 0N GROUND AND Filed Nov. 25, 1960 PLANE FED TAPERED FEED 1 2 sheets-sheet 2- IZOO MC/SEC E- PLANE W Fl 13-5 2000 MC/SEC v FIE-E H-PLANE IN V EN TORS ATTORNEY point.

United States Patent This invention relates to antennas, and more particularly to a broadband tapered array utilizing radiating elements whose lengths are functions. of element position in the array. The antenna with which this invention is concerned belongs to a family known as Pseudo-Infinite, Logarithmically Periodic (PILP) Antennas.

PlLP antennas have the desirable characteristic of maintaining a relatively constant radiation pattern and impedance over indefinitely large bandwidths. However, because of the asymmetry or complementary nature of the radiating components, no satistactory means has been found for exciting such antennas in conjunction with the counterpoise or ground plane. As a result, PILP' antennas have generally been restricted to optimum performance only in an essentially free space environment.

Our invention concerns a 'PILP-type antenna Whidh operates with a ground plane as an essential part of the antenna. Briefly, the antenna physically comprises a series of radiating elements whose lengths and spacing-s increase uniformly with increasing distance from each element to a feed point, and where the elements are shuntexcited by one-half of a tapered two-wire transmission line. The transmission line is developed from a coaxial line and the point of transition from the coaxial line to the half of the twin-wire feed line is called the feed The ground plane comprises a suitable planar conducting surface having dimensions large compared to those of the radiating elements, and may comprise the surface of the earth or a conducting sheet. The outer conductor of the coaxial line is electrically connected to the ground plane or connterpoise at the feed point and the inner conductor is extended tobecome half of a tapered two-wire transmission line which is electromagnetically coupled to successive elements or radiators at increasingly greater distances above the ground plane. In short, the inner conductor diverges trom the ground plane and this is what is meant by tapered.

An object of our invention is the provision of a pseudoinfinite logarithmically periodic antenna that is capable of operating in conjunction with the ground plane.

Other objects of our invention will become apparent from the following description of a preferred embodiment 7 thereof, reference being had to the accompanying drawings in which:

FIGURE'I is an isometric view of an antenna and ground plane embodying our invention;

FIGURE 2 is a greatly enlarged fragmentary sectional view of a portion of FIGURE 1;

FiGURE 3 is a transverse section of the antenna taken on line 33 of FIGURE 2;

FIGURES 46, inclusive, are radiation patterns of the antenna over a 3 to 1 frequency range; and

FTGURE 7 is a plot of voltage standing wave ratio against frequency for an antenna embodying our invention.

Referring now to the drawings, a preferred embodiment of the invention comprises an antenna 10 having conducting elements Ill-'16, inclusive, arranged in a row 3 ,101,474 Patented Aug. 20, 1963 ice vhaving real dimensions substantially greater than the dimensions of plates 114.6 and may comprise the surface of the earth, a counterpoise, or a conducting sheet such as copper plate.

Plates 1146, inclusive, have lower edges 11a-16a, respectively, which intersect and make electrical contact with surface 20. The upper edges lib-16b are inclined relative to surface 20 so that the lengths of successive plates increase in accordance with a predetermined ratio in a direction from one end of the antenna to the other, i.e., from the left to the right as viewed. The widths of successive plates and the interplate spacings as measured along the axis of the antenna, likewise increase in accordance with a predetermined ratio. In sum, the physical dimensions and spacings of conducting elements 11-16 vary monotonically from a minimum at one end of the antenna to a maximum at the other end in progressive increments of a predetermined ratio.

While theoretically an antenna embodying this invention could be constructed having a semi-infinite bandwidth, in practical form the antenna is suited to operate over a given fi-nite frequency of range, the length L /4 of the largest radiating element 16 roughly corresponding to one-quarter of the largest operating Wavelength of the required band. The lengths and separations of the remaining elements are unifiormly decreased or tapered to vanishingly small dimensions so that essentially there is no smallest operating wavelength, and all other dimensions of the elements are kept in constant ratios to the lengths. The antenna characteristics, then, with respect to directivity, beam shape and impedance, remain substantially the same tor wavelengths less than L /4 because a part of the antenna has physical dimensions corresponding to any operating frequency within that range. Because antenna arrays are discrete structures, the frequency bandwidths of individual radiating elements are chosen to overlap in order that the periodicity of radiation patterns with changes in frequency will be minimized or eliminated entirely.

The number and size of radiating elements used in any one antenna depends upon the frequency range over which the antenna is operative. While the size of the smallest element 11 is dictated by the upper limit of the operating range, in practice it was determined that a tapered extension 21, indicated in broken lines in FIG URE 2, improved the impedance match of the antenna to the feed line and thus optimized antenna performance. The extension 21 may be a solid plate, or may, alternatively, comprise successively smaller radiating elements 21a, 21b and lie which are spaced progressively closer together.

- In order to feed electromagnetic wave energy to the antenna (if it is used as a transmitter) or'to receive intercepted waves from the antenna (when it is used as a receiving antenna) over a broad range of frequencies,

a relatively frequency-insensitive feed system is emly connected to surface 20 and the inner conductor 24 is supported in and extends through a dielectric Washer 26 secured in a suitable opening 27 in the conducting surface 29. The extension 24a of conductor 24 above surface 2i is bent over at the feed point and extends from this bend for the hull length of the antenna closely spaced to the conducting elements 11-16. The conductor 24a is tapered, i.e., its spacing from surface 2i) increases progressively from the feed point F to the last conducting element 16 so that (the conductor makes an angle with the conducting surface. The conductor 24:: is electromagnetically coupled to and insulated from each of the conducting elements lli.6, and in order to support the conductor in this position relative to the conducting elements, insulator brackets 30, see FIGURE 3, through which the conductor extends, are secured to the sides of the plates.

A principal advantage of the above described counterpoise antenna is that it is ideally suited to operation in conjunction with a conducting surface or ground plane. Indeed, the ground plane is an essential element in the entire combination. At any particular frequency, the off-resonant ones of the conducting elements ll16 of the antenna 10 are practically uncoupled and contribute little to the operation of the antenna. This is believed to account for radiation patterns that are essentially constant and practically free of disturbing lobes or pattern break up.

By way of example, an antenna embodying our invention and having the following dimensions and operating characteristics was built and successfully operated:

Area of ground plane 20 20" x 40" Number of conducting plates 6 Distance from feed point F to remote edge of largest conducting element 16 16" Angle of inclination of top edges of plates degrees 20.5 Plate Dimensions:

Width, Mean Spacing, inches Height, inches inches Plate 11 and Extension 21 2%; 9A6 Plate 12 at 1 I Zia Plate 13 is 1 4 Plate 14-" 1% 2% 1M6 Plate 15 1 4; 3 1946 Plate 16 2% 2%;

Coaxial cable ohms 5O Frequeincy range mc 600-2000 Average VSWR 2.75

The performance of the above described antenna is shown in part in FIGURES 4-7, inclusive. The E- and H-plane radiation patterns cover a frequency range of 600 to 2000 megacycles per second, and additional bandwidth is achieved by increasing the numberoi conducting elements. The E-plane patterns shown in FIGURES 4a, 5a and 6a are those patterns in which the electric polarization vector is orthogonal to the ground plane and which would correspond to vertical cuts in the radiation pattern of the large structure mounted on the earths surface. The H-plane patterns are taken in a plane orthogonal to the E-plane and inclined 10 degrees to the ground plane. The ground plane intersects the E-plane patterns at zero degrees. These patterns show a wellformed beam :of about 33 degrees half-power beamwidth in the E-plan'e and 118 degrees beamwidth in the H-plane. This corresponds toabout 10 :db directive \gain above an isotropic radiator, and gain and pattern shape are almost I constant over the entire frequency range. The beam is symmetric in the H-plane, and in the E-plane is inclined about 25 to 30 degrees to the-ground plane. Crosspolarized radiation is indicated by broken lines in the 6 figures. The voltage standing wave ratio plots 35 her the antenna described above are shown in FIGURE 7. This VSWR can be optimized and an improved match can be achieved with a coaxial line characteristic impedance of 50 ohms; the nominal antenna impedance in the latter situation is about 35 ohms. Input impedance can be controlled by choice of position and size or" the tapered transmission line.

Changes, improvements and modifications of the above described embodiment of our invention may be made by those skilled in the art without departing from the precepts and spirit of the invention. The scope of the invention is defined in the appended claims.

We claim:

1. In combination, a ground plane and a broadband antenna having an axis comprising a plurality of parallel axially spaced conducting elements projecting away from said ground plane, each of said elements being electrically connected to said ground plane, the linear dimensions and spacings of successive ones of said elements increasing in a direction from one end of the antenna to the other, a two-conductor transmission line for feeding electromagnetic wave energy to said elements, one of said conductors being electrically connected to said ground plane, the other of said conductors extending the full length of the antenna closely spaced to and insulated from said conducting elements, said other conductor traversing said conducting elements at progressively increasing distances from the ground plane ina direction from said one end of the antenna to the other.

2. A broadband antenna comprising a planar conducting surface, a plurality of parallel spaced conducting elements arranged in a row in a first plane extending normal to said surface, said elements being electrically connected to said surface, the linear dimensions and spacings of said elements increasing in a direction from one end of the row to the other end, and a coaxial line having an outer conductor and an inner conductor, said outer conductor being electrically connected to said surface, said inner conductor having an extension parallel to said first plane and spaced from and electromagnetically coupled to said elements, the spacing between said extension of the inner conductor and said surface increasing in a direction from said one end of the row to the other.

3. In combination with a ground plane, a broadband antenna comprising a plurality of parallel spaced conducting plates arranged in a row in a first plane extending normal to said ground plane, said plates intersecting and being electrically connected to said ground plane, the linear dimensions of and spacings between said plates increasing in a direction from one end of the row to the other, the edges of said plates opposite from ground plane lying in a plane diverging from said ground plane in a direction toward said other end of the row of plates, and a coaxial line having an outer conductor and an inner conductor, said outer conductor being electrically connected to said ground plane, said inner conductor having an extension disposed parallel to said first plane and being spaced from and electromagnetically coupled to said plates, the spacing between the extension of said inner conductor and said ground plane increasing from a minimum at said one end of the row to a maximum at the other.

References Cited in the file of this patent UNITED STATES PATENTS 2,981,951 Wickersham et a1. Apr. 25, 1961 

2. A BROADBAND ANTENNA COMPRISING A PLANAR CONDUCTING SURFACE, A PLURALITY OF PARALLEL SPACED CONDUCTING ELEMENTS ARRANGED IN A ROW IN A FIRST PLANE EXTENDING NORMAL TO SAID SURFACE, SAID ELEMENTS BEING ELECTRICALLY CONNECTED TO SAID SURFACE, THE LINEAR DIMENSIONS AND SPACINGS OF SAID ELEMENTS INCREASING IN A DIRECTION FROM ONE END OF THE ROW TO THE OTHER END, AND A COAXIAL LINE HAVING AN OUTER CONDUCTOR AND AN INNER CONDUCTOR, SAID OUTER CONDUCTOR BEING ELECTRICALLY CONNECTED TO SAID SURFACE, SAID 